This application relates to a method of repairing a full hoop structure with a welding process, wherein heat treating is performed both at the location of the weld, and at a diametrically opposed location.
Welding methods are sometimes necessary to repair metallic structures. As an example, a cast part may have a defect such as shrinkage that may have occurred in a cast mold. Alternatively, small cracks may form in the part.
Such operations are often used in components for gas turbine engines. Structures that could be called “full hoop,” or structures that surround a central axis for 360°, often require such welding procedures. Examples of such parts in a gas turbine engine would be a diffuser case and a turbine exhaust case.
The weld being performed at a location on the part may cause an unacceptably high residual stress. In the prior art, this stress has been relieved by some post-weld heat treatment.
In one prior art method, the entire structure has been heated isothermally to heat-treat temperatures. Heating isothermally does not induce additional thermal stress at the weld, so the residual stress remains constant until actual heat treatment takes place. This “global” heating can affect dimensions that have been “machined” into the part by causing their residual stresses to also relax. In many cases, it has not been found practical due to cost and complexity to fixture the part during heat treatment to hold these dimensions constant.
Thus, localized heat treatment has also been utilized to avoid loss of dimensions. Local heat treatment can have unforeseen and potentially detrimental effects on the intended stress relaxation. The region being heated locally will expand due to its temperature change. The surrounding non-heated material will resist this expansion causing the heated area to become more compressively loaded. Since the residual stress due to weld is tensile, the net effect of local heating is to temporarily reduce the value of the tensile stress in the weld. If sufficient care is not exercised, it is possible to reduce the value of the tensile stress so much so as to eliminate it completely. In this case, subsequent heat treatment for stress relaxation would be ineffective since the stress would already be reduced to zero. Note that the full value of the residual stress in this case would return when the locally applied temperature was removed.
Also of concern, would be a situation in which the weld stress was reduced by local heating through zero and into a state of compression. This stress would relax during subsequent heat treatment, but this is far from the original intent of the heat treatment process, which was to reduce the tensile residual stress associated with the weld.